Britain’s opposition Labour Party said Prime Minister Theresa May’s government could face an action for contempt of parliament after only publishing on Monday a summary of the legal advice it received on her Brexit deal with the European Union.

The threat of contempt proceedings is another hurdle for May to clear before parliament votes on Dec. 11 on her deal for Britain’s exit from the European Union, its biggest shift in foreign and trade policy for more than 40 years.

Labour argues that that vote is so important for the future of the country that lawmakers should be able to see any detailed legal warnings concerning parts of the withdrawal agreement.

It has threatened to initiate proceedings against the government for contempt of parliament, a move that could potentially result in one or more ministers being suspended or expelled from the House of Commons, the lower chamber.

Earlier on Monday, the government published a 43-page summary document saying the deal meant Britain could be locked into a full customs union with the EU to prevent a hard border on the island of Ireland unless a new trade accord is signed.

“This falls far short of what parliament demanded (in a legally binding vote last month). It is not the full legal advice,” a Labour Party official said. “Ministers should be aware that they are treading on very thin ice.”

The government, in line with usual practice, has resisted publishing its full legal advice on grounds of confidentiality.

Supporters of leaving the EU have demanded that there must be a way for Britain to unilaterally leave a full customs union with the EU in order to allow the government to strike trade deals with other countries such as the United States.

But the legal advice says the so-called “backstop”, or guarantee of no hard border between EU member Ireland and UK-governed Northern Ireland, “will continue to apply unless and until it is superseded” by new trading arrangements with the EU.

Under parliamentary rules, it is up to the Speaker, who chairs debates in the House of Commons, to decide whether to allow a contempt motion to be voted upon.

If it passes, it would then be referred to a committee which would rule on whether contempt had taken place. If so, it would then recommend a punishment, which lawmakers must agree.